1. Field of the Invention
The present invention relates to a phase-change optical information recording medium which includes a recording layer located on a substrate and in which information can be recorded and erased. In particular, the present invention relates to a phase-change optical information recording medium in which information can be recorded and erased utilizing reversible phase-change of the recording layer between an amorphous phase and a crystal phase. In addition, the present invention also relates to an information recording and reading method using the phase-change optical information recording medium.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 8-124218 discloses a phase-change optical information recording medium in which information can be recorded and erased utilizing mark edge recording. The disclosed optical recording medium is prepared by forming a first dielectric layer, a recording layer, a second dielectric layer, a reflecting layer, a third dielectric layer, and a UV curable resin layer on a substrate in this order. The reflecting layer is formed of a transparent ultra-thin film of a metal, Si, or Ge. The third dielectric layer is formed of a dielectric material having a refractive index n more than 1.5. The medium satisfies the following relationship:
Ac greater than Aa 
wherein Ac represents the absorptance in a crystalline state and Aa represents the absorptance in an amorphous state.
In this known art, an optical information recording medium having a crystalline absorptance Ac larger than the amorphous absorptance Aa (that is, the recording medium having a crystalline reflectance smaller than the amorphous reflectance) is provided. However, the recording medium has a drawback in that the medium does not have compatibility with CD-ROMs in view of reflectance.
Japanese Laid-Open Patent Publication No. 11-39716 discloses a medium having such a large refractive index as 2.2 to 2.3; however, the reflectance and characteristics of the medium having such a large refractive index widely change because the variation of the film thickness seriously influences on the characteristics.
Accordingly, an object of the present invention is to provide a phase-change optical information recording medium which can perform high density recording using light having a relatively short wavelength such as blue light and which can perform recording and reading while having compatibility with CD-ROMs.
Another object of the present invention is to provide an information recording method by which information can be recorded in the recording medium at a high density.
According to an aspect of the present invention, there is provided a phase-change optical information recording medium in which information can be recorded, erased and read and which includes a substrate and a recording layer located overlying the substrate. The recording layer achieves a crystal phase and an amorphous phase, and the light energy absorptance Ac of the recording layer in the crystal phase is lower than the light energy absorptance Aa thereof in the amorphous phase. Information is preferably recorded in the medium with light having a wavelength of from 370 to 450 nm at a recording pitch of from 0.3 xcexcm to 0.52 xcexcm.
This recording medium can perform high density recording, and in addition since the refractive index of the recording layer in the crystal phase is higher than that in the amorphous phase, the medium has compatibility with CD-ROMs.
Information is preferably recorded in the phase-change optical information recording medium while maintaining the ratio of the recording pitch to the recording beam diameter (1/e2) in the range of from 0.5 to 0.9, and more preferably from 0.6 to 0.85. When the ratio is too small, the adjacent track tends to be erased, resulting in increase of errors. To the contrary, when the ratio is too large, the recording capacity of the medium decreases.
This recording medium can perform high density recording, and in addition since the refractive index of the recording layer in the crystal phase is higher than that in the amorphous phase, the medium has compatibility with CD-ROMs.
In addition, information is preferably recorded at a recording density of from 0.05 to 0.16 xcexcm/bit, and more preferably from 0.11 to 0.16 xcexcm/bit. When the recording density is less than 0.11 xcexcm/bit, the signal amplitude decreases, resulting in increase of errors.
This recording medium can perform high density recording, and in addition since the refractive index of the recording layer in the crystal phase is higher than that in the amorphous phase, the medium has a compatibility with CD-ROMs.
The phase-change optical information recording medium of the present invention preferably includes a lower protective layer located between the substrate and the recording layer, an upper protective layer located on the recording layer, a heat dissipation layer located on the upper protective layer, and a UV curable resin layer located on the heat dissipation layer. The refractive index of the lower and the upper protective layers is preferably from 1.8 to 2.19.
In this recording medium, information can be recorded even with such a short wavelength as about 400 nm, e.g., blue light.
The recording layer preferably includes Ag, In, Sb and Te so that the information can be highly sensitively recorded and read, and overwritten in the medium.
The substrate preferably has grooves with a depth of from 25 nm to 50 nm, and more preferably from 25 to 35, so that the information can be recorded and read with stable tracking even when using short wavelength light. When the depth is too large, the difference in reflectance between the recording medium and CD-ROMs increases, and therefore, it becomes hard to detect signals.
The recording layer preferably has a thickness of from 7 nm to 20 nm, and more preferably from 7 to 15 nm, so that information can be recorded and read with stable tracking even when using short wavelength light. When the thickness is too large, the jitter deteriorates, resulting in increase of errors.
Information is preferably recorded therein and read therefrom at a line speed of 1.2 m/s to 14.0 m/s so that high speed recording (overwriting) can be performed even when using short wavelength light.
In the another aspect of the present invention, an information recording method is provided which includes the steps of providing a phase-change optical information recording medium comprising a substrate and a recording layer located overlying the substrate and achieving a crystal phase and an amorphous phase, wherein the recording layer satisfies the following relationship:
Ac less than Aa 
wherein Ac represents an absorptance of the recording layer in the crystal phase against light having a wavelength of from 370 nm to 450 nm and Aa represents an absorptance of the recording layer in the amorphous phase against the light having a wavelength of from 370 nm to 450 nm; and recording information in the recording layer using light having a wavelength of from 370 to 450 nm at a recording pitch of from 0.3 to 0.52 xcexcm.
In yet another aspect of the present invention, an information recording and reading method is provided which includes the medium providing step and the information recording step mentioned above, and reading the information with light at a speed of from 1.2 m/s to 14.0 m/s.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.